1. Field of the Invention
The present invention generally relates to a Multiple-Input Multiple-Output (MIMO) system, and more particularly to a random beamforming method for the MIMO system.
2. Description of the Related Art
Next-generation mobile communication (4G) requires a significantly higher transmission rate than that of third-generation mobile communication (3G). To satisfy the required transmission rate, both a base station at a transmitting side and a terminal at a receiving side may utilize a Multiple-Input Multiple-Output (MIMO) system adopting multiple antennas.
The MIMO system is a communication system for transmitting and receiving data using multiple transmit and receive antennas, and divides a MIMO channel formed by the transmit and receive antennas into a plurality of independent spatial channels. Each spatial channel is mapped to one dimension. When additional dimensions generated by the multiple transmit and receive antennas are exploited, the performance of the MIMO system is further improved.
Using the MIMO system, data transmission schemes can be a Spatial Multiplexing (SM) scheme for transmitting data at a high rate without increasing a system bandwidth by simultaneously transmitting different data using multiple antennas at the transmitting and receiving sides and a Spatial Diversity (SD) scheme for transmitting identical data through multiple transmit antennas and obtaining a transmit diversity gain.
The MIMO transmission scheme can be divided into an open-loop scheme and a closed-loop scheme according to the use of channel gain information at the transmitting side. Because the transmitting side does not need channel information in the open-loop scheme, it can be easily implemented and does not perform a channel information feedback process. Open-loop transmission schemes include a well-known Space-Time Block Code (STBC) scheme, a spatial multiplexing scheme based on Vertical Bell Labs LAyered Space-Time (VBLAST) decoding, etc. On the other hand, the closed-loop scheme employs channel information fed back from the receiving side and provides better throughput if the channel information is correct. However, because an amount of channel information may increase to provide a feedback of the correct channel information, the amount of feedback information and the system throughput have a trade-off relation.
A random beamforming technique is a transmission scheme in which multi-user diversity and scheduling are combined. Theoretically assuming that an infinite number of users are present within a cell, the performance of complete closed-loop beamforming can be provided using only a small amount of feedback information.
In the random beamforming technique, the transmitting side does not need channel information and performs beamforming with a randomly generated weight. Receiving sides notify the transmitting side of their Signal to Interference plus Noise Ratios (SINRs) or allowable transmission rates based thereon using only a limited amount of feedback information. The transmitting side allocates a channel to the receiving side with the highest SINR (or transmission rate) in the current weight. Under a fast fading environment with fast channel variations, channels may be evenly allocated to many users. However, there is a problem in fairness if a weight is fixed under a slow fading environment with slow channel variations. In this case, if a weight can be newly changed to an arbitrary value at a short time interval, the effect of fast channel variations can be obtained and the fairness between multiple users can be improved. The beamforming technique can be exploited in combination with a scheduling algorithm such as proportional fairness scheduling that simultaneously considers efficiency and fairness by referring to an amount of currently received data, and first allocating a channel to a user with a small amount of received data rather than a user with a large amount of received data, as compared with a channel allocation method using only a momentarily varying SINR.
The beamforming technique is a method for obtaining a transmit diversity gain using multiple antennas at a transmitting side. Recently, research has been being conducted on the random beamforming technique for the MIMO system adopting multiple antennas also at a receiving side.
FIG. 1 schematically illustrates a structure of a communication system to which a multi-antenna random beamforming technique is applied. In FIG. 1, terminals also have at least two antennas, respectively. The conventional multi-antenna random beamforming technique aims at performance approximating that of Singular Value Decomposition (SVD) multiplexing capable of maximizing a channel capacity of a single-user MIMO system. For this, the multi-antenna random beamforming technique exploits both multi-user diversity and receive diversity on the basis of multiple antennas of the terminals.
In FIG. 1, a base station 101 generates an arbitrary weight vector V0, multiplies V0 by an input signal vector x, and transmits V0 x through multiple transmit antennas.
Among a plurality of terminals 103, 105, and 107, an arbitrary k-th terminal receives a signal based on a channel matrix Hk. At this time, it is assumed that the terminal exactly knows the channel matrix. To exploit the well-known SVD multiplexing capable of obtaining a high channel capacity, SVD of the channel matrix Hk is taken and a matrix Uk configured by left singular vectors is computed and multiplied by the received signal. Thus, the received signal can be expressed as shown in Equation (1), where nk is the additive Gaussian noise vector.
                                                                        r                k                            =                                                U                  k                  H                                ⁡                                  (                                                                                    H                        k                                            ⁢                                              V                        0                                            ⁢                      x                                        +                                          n                      k                                                        )                                                                                                        =                                                                                          U                      k                      H                                        (                                                                  U                        k                                            ⁢                                                                        ∑                          k                                                ⁢                                                  V                          k                          H                                                                                      )                                    ⁢                                      V                    0                                    ⁢                  x                                +                                                      U                    k                    H                                    ⁢                                      n                    k                                                                                                                          =                                                                    ∑                    k                                    ⁢                                                            V                      k                      H                                        ⁢                                          V                      0                                        ⁢                    x                                                  +                                                      U                    k                    H                                    ⁢                                      n                    k                                                                                                          (        1        )            
If the number of linked terminals is sufficiently large, the probability in which a terminal with Vk significantly similar to V0 is present increases. Due to Vk″V0≈I, the effect of SVD multiplexing can be obtained. In this case, the channel capacity can be further increased when a water-filling algorithm is applied.
The terminal computes SINRs of respective data symbols, performs conversion to a total transmission rate at which transmission is possible, and transmits information to the base station through a limited feedback channel. At this time, the total transmission rate is expressed as shown in Equation (2) where Γki is an SINR of an i-th symbol xi computed in the k-th terminal.
                              C          k                =                              ∑            i                    ⁢                                    log              2                        ⁡                          (                              1                +                                  Γ                  ki                                            )                                                          (        2        )            
The base station allocates all channels to an
      arg    ⁢                  ⁢                  max        k            ⁢                          ⁢              {                  C          k                }              -  thterminal and repeats the above-described procedure after a predetermined time elapses.
However, if the number of linked terminals in a cell is infinite in the conventional random beamforming method, a terminal with Vk close to V0 is present because it is difficult that Vk and V0 exactly match in a limited number of users, Vk″V0≠I. That is, self-interference occurs when x is recovered. Assuming that every terminal uses two receive antenna, x will be a 2×1 vector. At this time, the SINR of x1 is given as shown in Equation (3).
                              Γ                      k            1                          =                                                            [                                  ∑                  k                                ]                            11              2                        ⁢                                                                                                [                                                                  V                        k                        H                                            ⁢                                              V                        0                                                              ]                                    11                                                            2                        ⁢                          E              s                                                                                            [                                      ∑                    k                                    ]                                11                2                            ⁢                                                                                                            [                                                                        V                          k                          H                                                ⁢                                                  V                          0                                                                    ]                                        12                                                                    2                            ⁢                              E                s                                      +                          N              0                                                          (        3        )            
In Equation (3), Es is the average symbol energy of x and No is the noise spectral density.
Because [Σk]112|[Vk″V0]12|2Es of Equation (3) acts as interference, performance degradation occurs.
Because the conventional random beamforming method must allocate all independent channels to one terminal, the probability in which one arbitrary terminal with the best combination of channels is present is very low when good and bad channels are evenly mixed for every terminal. Thus, it may not be expected that the base station can always transmit data at the best transmission rate. The base station needs to improve transmission efficiency by dividing and allocating channels for multiple users.